SpudFiles

This is my latest gun that I have designed and almost finnished. It will be semi auto. It works by having equal pressure on both sides of the piston (which at this point is covering the hole to the barrel) . You then open the ball valve which releases the pressure from one side causing the piston to move to that side and opening the hole to the barrel. When you close the ball valve the pressure builds back up on valve side and pushes the piston back over the hole
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looks good, how are you reloading your ammo? It seems a bit of a waste to have a semi auto valve if you still have to load the ammo manually.

You may want to put some sort of stoppers on both sides of the piston so that it will line up exactly each time.

As far as the trigger goes, I would suggest something other than a ball valve. A blowgun would be nice, or maybe even a hammer valve, especially since you're wanting to shoot semi-auto.

Now on to efficiency. Cut down on the pilot volume. I sure if the piston seals nicely, you will be able to significantly cut the pilot volume and still create what you want.

The concept looks good...now finish the cannon and let's see it in action.

Looking good, except you have a bit of dead volume, and when the piston moves to one side, your expanding the chamber, so you give the air more room to expand, loosing pressure.

With that and the dead volume you may end up loosing a bit of power, considering it's semi auto it could be a noticeable amount of power.

It also doesn't look like it'll be very rapid, you've got to open the valve, close it, build up pressure, and repeat. And you don't have a hopper design yet either, and they can be quite hard to implement without more then one projectile (if it's spherical) getting sucked into the barrel.

I doubt it will be worth the hassel unless it's large scale, you may as well use a sprinkler valve if it's small scale, essentially you're doing the same thing except a sprinkler valve comes pre-made and is a lot easier to work.

The question is...

Why not go with a standard chamber sealing piston valve with a check valve/equalisation hole in the piston?

Essentially thats what you have there but with the equalisation hole replaced by an external tube.

looks a lot like typical, you could just put a equalization on the piston and have one input to the pilot, to conserve air you an just put a 3 way valve for the input.

To jack, i will be setting up a hopper and the picture is not to scale in tems of pilot volume and such. I will probably go to a diferent type of valve, it was just easy to draw a ball valve. To what hotwire said, i thought that i might try something different "the only way you learn is from mistakes".